1. Field of the Invention
This invention relates to light emitting diodes, and more particularly to a capacitor-regulated high efficiency driver for a light emitting diode.
2. Description of the Related Art
A light emitting diode (LED) driver in a photoelectric transceiver (e.g., an infrared IR transceiver) is critical to the transmitting operation of the transceiver. The LED driver requires the highest operating current of any component in the transceiver. Typically, the peak current of the LED driver can reach 500 mA or more. Since many infrared transceivers are battery operated, the current draw of the LED driver is an important concern.
Therefore, enhancing the efficiency of the LED driver to reduce power consumption would be a considerable advantage, particularly where the IR transceiver is used in telephone or computer network applications. While in data communications applications the IR transceiver is in the receive mode most of the time, in telephone or synchronized network applications the transceiver is typically in the transmit mode for most of its operating time.
FIG. 1A illustrates a conventional switching LED driver, comprising an external resistor R and an N-type field effect transistor (FET) N. When the gate of the FET is low, the FET is turned off, and no current flows through the LED. When the gate of the FET is high, the FET is turned on, and a current flows between Vdd and ground to illuminate the LED. The FET has a very low on-resistance, usually less than 0.1 ohm. Thus, the current depends on the forward voltage Vf of the LED and the resistance of an external resistor R:
ILED=(Vddxe2x88x92Vf)/(Ron+R)xe2x80x83xe2x80x83(1)
where
ILED is the current through the LED,
Vf is the forward voltage of the LED, and
Ron is the on-resistance of the FET.
This circuit is relatively simple, and provides the additional advantage that the IR transceiver internal power consumption excludes the power consumption of the resistor R. However, from the standpoint of power conservation there are significant disadvantages to this circuit. For example, the LED driving current depends on the voltage Vdd, which is not controllable. Further, the power efficiency of the circuit is low because the power consumption of the resistor R constitutes a large portion of the power provided by Vdd.
A simple calculation shows that the ratio of the power consumption on resistor R to the total power supplied by Vdd is
PR/PO=(R*(Vddxe2x88x92Vf))/(Vdd*(Ron+R))xe2x80x83xe2x80x83(2)
where
PR is the power consumption of resistor R, and
PO is the total power provided by Vdd.
Usually R greater than  greater than Ron, so equation (2) can be reduced to
PR/PO=(Vddxe2x88x92Vf)Vdd=1xe2x88x92Vf/Vdd.
Thus, for example, if Vdd=5 V and Vf=2 V at 500 mA, this ratio is 3/5 which means more than one half of the total power supplied by Vdd is dissipated by resistor R.
The dependency of the LED driving current on Vdd can be seen in the following example. If Vdd ranges from 4.5 V to 5.5 V, Ron is 0.1 ohm, and an LED driving current ILED of 500 mA is required, the resistance of the resistor R should be 4.9 ohms to generate the LED driving current ILED of 500 mA at 4.5 V, which is the low value of Vdd. At 5.5 V, the high value of Vdd, the current would increase to 700 mA if Vf remains constant. Thus, fluctuations in the supply voltage Vdd directly affect the LED driving current, and the optical intensity of the LED will vary accordingly.
FIG. 1B shows a conventional pulsed current source (PCS) LED driver which holds the LED current constant, independent of Vdd. However, in this circuit, the IR transceiver internal power consumption is very high. The ratio of the power consumption PPCS of the pulsed current source to the total power provided by the power supply Vdd is
PPCS/PO=1xe2x88x92Vf/Vddxe2x80x83xe2x80x83(3)
Thus, in this circuit, the IR chip internal power consumption is very high, which causes significant thermal problems.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a driver circuit for a light emitting diode, which overcomes the above problems.
Another object is to provide a high efficiency LED driver circuit which utilizes a capacitor to regulate the LED driving current, which provides high efficiency and low power consumption. The capacitor provides the LED driver current by discharging through the LED during transmission intervals, and the power supply for the device maintains the capacitor voltage. The capacitor charges to a preselected high threshold voltage, which determines the maximum LED optical emission intensity. As the charge depletes during transmit intervals, the capacitor discharges to a preselected low threshold voltage, at which point the capacitor pump controller opens a charge path from the power supply to recharge the capacitor.
The LED driver circuit of the invention accordingly operates at high efficiency with low power consumption. Moreover, the LED driver current can be regulated by changing the low threshold voltage and the high threshold voltage, thereby to control the optical intensity of the LED.
In a first aspect, the present invention provides a driver circuit for a light emitting diode in an optical transmitter which converts an electrical signal into an optical signal. The driver circuit includes a capacitor having one side connected to a higher voltage terminal of a power supply through a charge switch, a second side of the capacitor being connected to a lower voltage terminal of the power supply, the anode of the light emitting diode being connected to a connection point of the charge switch and the capacitor, the cathode of the light emitting diode being connected to a lower voltage terminal of the power supply through a discharge switch, and a controller for closing the charge switch to charge the capacitor when a voltage on the capacitor reaches a preset low voltage threshold and for opening the charge switch when the voltage on the capacitor reaches a preset high voltage threshold. The discharge switch is closed responsive to an intensity of the electrical signal and the capacitor thereby discharges through the light emitting diode to emit an optical signal corresponding to the electrical signal.
In another aspect, an optical transmitter is provided, which includes a light emitting diode, and a driver circuit for activating the light emitting diode responsive to an electrical signal. The driver circuit includes a capacitor having one side connected to a higher voltage terminal of a power supply through a charge switch, a second side of the capacitor being connected to a lower voltage terminal of the power supply, the anode of the light emitting diode being connected to a connection point of the charge switch and the capacitor, the cathode of the light emitting diode being connected to a lower voltage terminal of the power supply through a discharge switch, and a controller for closing the charge switch to charge the capacitor when a voltage on the capacitor reaches a preset low voltage threshold and for opening the charge switch when the voltage on the capacitor reaches a preset high voltage threshold. The discharge switch is closed responsive to an intensity of the electrical signal and the capacitor thereby discharges through the light emitting diode to emit an optical signal corresponding to the electrical signal.
In further exemplary aspects of the invention, the discharge switch preferably includes an N-type FET having a drain connected to a cathode of the light emitting diode, a source connected to the lower voltage terminal of the power supply and a gate connected to the electrical signal.
Preferably, the controller includes a voltage comparator having a positive input connected to an output of the comparator through a resistor and connected to the capacitor through a comparator input resistor, an output signal of the comparator controlling the charge switch and a threshold voltage applied to the comparator determining the preset low voltage threshold and the preset high voltage threshold.
The controller may further include a voltage divider applying a divided voltage to the comparator input resistor. The voltage divider preferably includes a first N-type FET having a drain connected to the capacitor, and a source and gate connected to the comparator input resistor and to a drain of a second N-type FET, a source and gate of the second N-type FET being connected to the lower voltage terminal of the power supply.
Preferably, the charge switch includes a P-type FET having a source connected to the higher voltage terminal of the power supply, a drain connected to the capacitor, and a gate connected to the output of the comparator.
The present disclosure relates to subject matter contained in Canadian Patent Application No. 2,311,435, filed Jun. 13, 2000, which is expressly incorporated herein by reference in its entirety.